ATS doesn't read proxy.config.http.forward.proxy_auth_to_parent

"proxy.config.http.forward.proxy_auth_to_parent" should work when we want ATS relay "Proxy-Authorization" header to parent proxy or origin server.

Current ATS doesn't read this config and set it to 0 (disable) by default. As a result, HttpTransactHeaders::copy_header_fields always remove "Proxy-Authorization" when building requests to origin server.